This invention relates generally to an optical retarder, and more particularly to an optical retarder operable over a wide range of incidence angles.
Optical retarders are generally used in some manner to alter the relative phase of polarized light passing through the retarder. Optical retarders are particularly suited for use in applications where control over the polarization is required. Polarization of light generally refers to the restriction of electric (or magnetic) field vector vibrations to a single plane. The polarization direction of electromagnetic radiation is generally considered the direction in which the electric field vector oscillates back and forth. The polarization vector is orthogonal to the beam direction within the light plane.
Polarized light can assume a number of different forms. Where a light beam oscillates in only one direction at a given point the beam is said to be linearly (or plane) polarized. The direction of oscillation is the polarization direction. If the light beam has two orthogonal polarization directions which vary in phase by 90.degree., the beam is said to be elliptical or circularly polarized. Circular polarization occurs when the magnitude of the two oscillations are equal (i.e., the tip of the electric field vector moves in a circle). Elliptical polarization occurs when the magnitudes are not equal (i.e., the tip of the electric field vector moves in a ellipse). In contrast, the orthogonal oscillations for unpolarized light are on average equal with a randomly varying phase difference.
Linearly polarized light can be obtained by removing all waves from an unpolarized light beam except those whose electric field oscillate in a single plane. Optical retarders can be used, for example, to convert linearly polarized light to circular or elliptically polarized light. When used to control the polarization of light, retarders are commonly constructed to induce 1/2- and 1/4-wave retardations. Generally, such retarders are used to produce a desired relative phase delay between two linear components of the polarized light.
One typical use of an optical retarder is a compensator which is used to introduce a phase delay in incident light to correct for phase differences between two components of polarized light introduced by mechanical or optical displacement of other optical components in a system. In a liquid crystal display (LCD), for example, birefringence of a liquid crystal cell may cause the linearly polarized light to become slightly elliptical. A retarder is used to convert the elliptically polarized light back to linearly polarized light. The compensating retarder is placed in the light path and is tuned to a particular phase difference introduced by the birefringence of the liquid crystal.
Typical optical retarders are constructed of birefringent materials. The birefringent materials form a fast and slow path along two orthogonal in-plane axes of the retarder. When the axes of the birefringent retarder are aligned at 45.degree. degrees to the polarization direction of the incident light, the retarder can be used to introduce or compensate for phase differences between two polarization components. The fast and slow path of the birefringent retarder results from different refractive indices for light polarized along the in-plane axes of the retarder. Larger retardation differences between the two polarization axes are achieved by increasing the refractive indices difference between the two in-plane axes and/or increasing the thickness of the retarder. Thus, by controlling the thickness and refractive indices of the birefringent material in the retarder, the optical properties of the retarder can be controlled.
In addition to refractive indices for light polarized along the in-plane axes of the retarder, the refractive index for light polarized in the thickness direction may influence the performance of the retarder in a given application. Compensators used in LCD display technology, for example, must provide relatively uniform retardation of light which is incident on the compensator over a relatively large angle range. It has been proposed that widened viewing angle ranges for LCD displays are obtainable by employing retardation films which have controlled refractive indices for light polarized in the thickness direction.
Current attempts to produce retarders having uniform wide angle performance have proven to be expensive and difficult to manufacture and have only achieved limited success in obtaining uniform wide angle optical properties. Attempts to obtain uniform wide angle performance are varied and include, for example, shrinking the film in the direction perpendicular to the stretching direction at the time of stretching, controlling, by stretching, the birefringence of a raw film produced from a molten polymer or a polymer solution under an applied electric field, laminating a film produced under an electric field onto a conventional phase retarder, and the like. Such processes are typically quite complex and expensive and achieve only limited success. As the process and materials used in forming the birefringent portion of a retarder become more complex, it becomes increasingly difficult to incorporate such material into the retarder.